In the case of a watch crystal made of an organic material such as a plastic material, it is preferable to use an ultrasonic technique for welding this type of crystal to a watch middle part which is also made of plastic. Indeed, the ultrasonic welding technique allows the crystal to be secured to the watch middle part in a perfectly sealed and long-lasting manner. However, there are cases where the ultrasonic welding technique is not appropriate. This is particularly true when elements are sensitive to mechanical and thermal stresses. In such cases, it is preferable to use bonding techniques which do not result in any increase in temperature. However, bonding techniques raise other problems. They do not always guarantee a good quality seal and raise problems of wear over time. Moreover, after bonding the parts, a polymerisation time has to be observed, which is usually twenty-four hours. There is a thus an interruption to the flow of production, which is detrimental from an economic point of view.